This proposal is the A1 submission for competitive renewal of R01 HL65352, Alveolar Epithelial Cells: Development and Repair. Our previous studies showed that the sub-population of alveolar epithelial type 2 cells (AEC2) that proliferate following oxidative lung injury exhibit a significant increase in telomerase activity, which recapitulates the elevated activity observed in fetal and neonatal AEC2. We also showed that those same lung cells that exhibit increased telomerase expression following injury are significantly more resistant to oxidative damage than a normoxia-exposed population. Our more recent studies show that lung cells from a telomerase null model, the terc-/- mouse, are targets for such damage and exhibit both chronic levels of oxidative stress and evidence of imperfect repair. These observations are a phenocopy of the debilitation leading to disease in aged lung. Taken together, our published findings and the preliminary data presented in this proposal show that activation of telomerase and maintenance of telomere integrity support both lung cell resistance to damage and normal ability to repair, while aging, telomerase knockdown and telomere shortening compromise both these activities. These findings indicate that telomerase activity and properly maintained telomeres correlate with both a lung cell survivor phenotype and a youthful whole lung phenotype. The chronically stressed molecular profile of aged and terc-/- cells implies that they are much less capable of normal tissue maintenance and injury repair than youthful and telomerase competent cells. We speculate that alteration of telomere length and stress signaling in terc-/- and aged lung stem/progenitor cells may lay the foundation for the variety of lung diseases and conditions for which telomerase mutation and aging are both risk factors. Aging and telomerase dysfunction alter stem/progenitor cell integrity and function in distal lung epithelium. To test this hypothesis, three specific aims are proposed: 1) To determine the mechanism that underlies the chronic stress phenotype and hypersensitivity to injury of aged and terc-/- AEC2, 2) To determine how aging, telomerase knock down and/or changes in telomere length compromise bronchioalveolar stem cell (BASC) progenitor function in vitro and 3) To determine how aging and telomerase dysfunction alter the ability of stem/progenitor cells to repair distal lung epithelium in a novel, targeted AEC2 depletion model.